Geogrid and civil engineering structure comprising such a geogrid

ABSTRACT

The invention pertains to a geogrid including drawn, polymeric longitudinal straps which run parallel or substantially parallel to each other and polymeric transverse straps bonded to the longitudinal straps, with the crosswise elastic modulus of the transverse straps being less than 15%, preferably less than 8%, of the lengthwise elastic modulus of the longitudinal straps. Such geogrids have improved strength compared to conventional geogrids.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to a geogrid comprising drawn, polymericlongitudinal straps which run parallel or substantially parallel to eachother and polymeric transverse straps bonded to the longitudinal straps.

2. Description of the Related Art

Grids as such are known. In GB 2266540 a grid is described which is madeof fully stretched polymeric longitudinal and transverse straps bondedtogether by means of, e.g., partial fusion of the straps.

WO 94/26503 describes a grid of drawn, polymeric straps bonded togetherby melting the polymer in the area of contact between the longitudinaland the transverse straps. The melting of the polymer is accomplished byheating conductive particles situated directly underneath the surface ofthe straps in a high-frequency electromagnetic field. In this way it isensured that only the portion of the polymer used to effect the bondwill melt. The remaining polymer is hardly affected at all and so thestrength of the drawn straps remains substantially unaffected. The gridaccording to WO 94/26503 can, in principle, be subjected to heavy loads.

However, in actual practice it was found that in the case of heavyloads, e.g., such as occur in civil engineering structures (i.e.,structures to do with, int. al., hydraulic and road engineering), loadedlongitudinal straps will break at a significantly lower load and exhibita significantly wider breaking load distribution than might be expectedon the basis of the specifications of these straps and the bondingtechnique applied.

SUMMARY OF THE INVENTION

The object of the instant invention is to provide a grid such asdescribed in the first paragraph that is especially suited for use incivil engineering structures and that does not suffer the describedpremature failure. This is achieved by making use of transverse strapsof which the crosswise elastic modulus is less than 15%, preferably lessthan 8%, of the lengthwise elastic modulus of the longitudinal straps.Preferably, the crosswise elastic modulus is also more than 0.1%,preferably more than 1%, of the lengthwise elastic modulus of thelongitudinal straps

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In an especially advantageous embodiment it holds that the crosswiseelastic modulus of the longitudinal straps is less than 15%, preferablyless than 8%, of the lengthwise elastic modulus of the (drawn)transverse straps. It is further preferred that the crosswise elasticmodulus of the longitudinal straps is more than 0.1%, preferably morethan 1%, of the lengthwise elastic modulus of the (drawn) transversestraps.

It was found that premature failure probably results from anunfavourable interaction between the longitudinal and the transversestraps. An insight into this interaction will be provided with referenceto the example below.

Use is made of a geogrid where drawn, polymeric longitudinal andtransverse straps (having a width of 12 mm and always 30 mm apart) havebeen welded together at an angle of about 90 degrees over their entirecontact area. Because the straps are drawn, their molecular chains areoriented essentially in the longitudinal direction. As a result of thisorientation the straps have poorer mechanical properties (strength,elastic modulus, elongation at break) crosswise than lengthwise.

If a tensile force is exerted on a longitudinal strap, a certainlengthwise elongation will occur in said strap. In places where thelongitudinal strap is bonded to a transverse strap, this elongation willresult in a crosswise force being exerted on said transverse strap. Aswas stated, it is precisely in this direction that drawn straps are lessstrong. Hence when subjected to heavier loads, the transverse strap willsplit.

This splitting does not in itself constitute a major problem for thegeogrid. However, because the transverse strap and the loaded strap arebonded together over the entire contact area, the transverse strap'ssplitting or cracking will lead to a crack and/or a load peak in theloaded longitudinal strap. This crack in its turn will lead to thepremature failure of the loaded longitudinal strap.

Selecting transverse straps with a comparatively low crosswise elasticmodulus means that the transverse straps will be deformed along with thelongitudinal straps without splitting or cracking on the side where theyare welded to the longitudinal strap, and that the unfavourable effectdescribed will not occur.

Preferably, in the geogrids according to the invention use is made oftransverse straps (or longitudinal straps) which even when a tensilestrain is exerted on one or more of the longitudinal straps (ortransverse straps) of at least 90%, or even at least 95%, of thespecific strength of the longitudinal straps (or transverse straps) willco-deform without cracking or splitting. In this way optimum use is madeof the strength of the straps.

Geogrids generally are made up of a “lattice” of longitudinal andtransverse straps bonded together at an angle, preferably of between 80°and 100°. Especially preferred are geogrids where the straps are bondedtogether through the polymer of the straps themselves, since such gridscan be made comparatively easily without recourse to glue or otheradhesives. Moreover, because only a fraction of the polymer of thestraps is melted, the strength of the straps is affected hardly if atall. Preferably, only 5 to 100 μm, or even only 5 to 30 μm, of thepolymer is melted.

A highly suitable method for effecting the bonds in the grids accordingto the invention is the one where the straps are placed one on top ofthe other, pressed together, and heated using a radiation sourceemitting electromagnetic radiation, e.g., a laser, with the strap facingthe radiation source being transparent to the radiation and the materialat the point where the straps are bonded together absorbing saidradiation (to a high degree).

It was found that this technique makes it possible to produce a verystrong weld rapidly (e.g., in 10-20 milliseconds). The strength of thisweld can be as high as the strength of the employed straps. In otherwords, two straps which are in the same straight line and have beenwelded together at a point where they overlap (which overlap, e.g., isat least twice the width of the straps) using this technique will have(substantially) the same strength as a single continuous, untreatedstrap.

Also, the aforesaid absorption of the radiation may be either by thepolymer itself or by a pigment added to the polymer.

In a very simple embodiment the strap facing the radiation source iscomposed entirely of transparent material. In that case there areseveral alternatives. For instance, the strap facing away from theradiation source may be made of an absorbent material. Alternatively,the straps to be bonded will both be transparent and a (thin) layer,e.g., ink or a film or foil, of an absorbent material is providedbetween the straps.

It will be obvious that, in principle, any configuration is possible solong as there is a material absorbing the radiation at the point wherethe bond is to be effected and so long as the radiation is able to reachthis material.

Another suitable embodiment is the one where the strap facing theradiation source is made up of more than one component. Use may be made,e.g., of a bicomponent strap (width 12 mm; thickness 0.55 mm) oftransparent polyester (0.50 mm thick) and polyester (0.05 mm thick) towhich a pigment has been added or of which the optical properties havebeen changed. This strap can be bonded to itself or to another strap invarious ways, so long as the radiation is able to reach an absorbentsection via a transparent section.

One advantage of using the multi-component strap is that this strap canfunction both as an exposed and as an unexposed strap. This means thatduring production there is no need to provide two or more supply linesfor two or more different materials.

The thickness of both the absorbent section of the strap comprising twoor more components and an intermediate layer (foil or film) may be verysmall.

Preferably, this thickness is between 5 and 100 μm. However, whenselecting this thickness the degree to which the material absorbs theradiation will have to be reckoned with. For that reason there is noabsolute lower or upper limit.

Preferably, use is made of radiation having a wavelength of 600 to 1600nm. For this range a large number of often inexpensive and reliableradiation sources are available. Also, there are many pigments on themarket which have high absorption in this range, e.g., carbon black.

Lasers are highly suitable for use in the manufacture of the geogridsaccording to the invention. Unlike in the case of quartz lamps, theradiation emitted by lasers can be focused using simple means.Furthermore, lasers have a narrow band width (“wavelength window”), sothat absorption by the transparent polymer can be prevented entirely orsubstantially entirely. Lamps, on the other hand, have a comparativelywide spectrum, so that the emitted radiation will always comprisewavelengths which are absorbed by the transparent polymer. In many casesthis less desirable absorption will amount to about 35% of totalradiation energy. It holds for the invention that this absorptionpreferably does not amount to more than 15%.

To code the geogrids use may be made of transparent straps provided witha dye which absorbs certain portions of visible light and scatters orreflects others, but which is transparant to the electromagneticradiation by means of which the straps are welded one on top of theother.

The straps preferably are made of a thermoplastic polymer such aspolyamides and polyolefins. Polyester, more particularly polyethyleneterephthalate and copolymers comprising ethylene terephthalic moieties,is especially suitable. It also holds that the degree of drawingpreferably is greater than 2 and less than 7. Highly suitable strapshave been disclosed, int. al., in EP 711 649.

In addition to the aforementioned geogrids the invention pertains tocivil engineering structures and works, such as dike bodies, beds,slopes, and the like, which have been reinforced with the geogriddescribed above.

Within the framework of the present invention the term “strap” refers tobodies where one of the dimensions clearly dominates the two otherdimensions and of which the thickness preferably is in the range of 0.2to 2 mm and the width is in the range of 3 to 30 mm, preferably in therange of 5 to 15 mm. The width of the straps preferably is at least fivetimes their thickness. Given the heavy loads occuring in civilengineering structures, it is preferred that the lengthwise specificstrength of the straps exceeds 200 MPa, and preferably 300 MPa.

The crosswise elastic modulus is measured (at a temperature of 21° C.and a relative atmospheric humidity of 65%) by compressing the strap inthe thickness direction between a smooth steel plate and, positionedparallel to it, a steel plane having a width of 2 mm and a lengthseveral times greater than the width of the strap. The plane is situatedon the conical side of a symmetrical wedge having an imaginary pointwith an angle of 300 and is obtained by flattening this point (throughmilling), such that the plane is perpendicular to the plane of symmetryof the wedge. The strap is clamped in such a way that the longitudinaldirection of the wedge corresponds to the transverse direction of thestrap. The crosswise elastic modulus, E_(tr) (in GPa), can be calculatedas follows:$E_{tr} = {\frac{d}{w \cdot b} \cdot \frac{S_{test} \cdot S_{tot}}{S_{tot} - S_{test}}}$

wherein w (in m) is the width and d (in m) is the thickness of thetransverse strap, and b (in m) is the width of the plane at the bottomof the wedge (in this case 2 mm). S_(test), the stiffness of themeasuring device without a clamped strap, and S_(tot), the jointstiffness of the measuring device and the strap, are determined by theaverage slope of the force-impression curve between 750 and 2250 N. Thewedge's speed is 0.1 mm/min and its movement is halted as soon as aforce of 3000 N is reached. One advantage of this method is that theelastic modulus in the direction of thickness of the strap is also takeninto account in the measured value.

The lengthwise elastic modulus, E_(lg), and the specific strength of thestraps are measured in accordance with ISO10319. For the lengthwiseelastic modulus use is made of the 1% secant elastic modulus.

Apart from the elastic modulus, the cracking behaviour of the strapsprovides a useful indication of the suitability of transverse straps foruse in the grids according to the invention. Use is made of a steelcylindrical pin having a mass of 700 g, a diameter of 2 mm, and tipangle of 60°. The pin is dropped over three identical straps placed oneon top of the other from such a height that the pin's velocity will be1.5 m/s the moment it strikes the top strap (approximately in thecenter).

The depth of penetration is controlled by a stop to about twice thethickness of a single strap. Next, the length of the crack in the topstrap is measured. The average crack length is determined by carryingout the experiment ten times and averaging the lengths found. It turnedout that transverse straps having an average crack length of less than60 mm and preferably of less than 40 mm are highly suitable for use inthe geogrids according to the invention.

It should be noted that in non-prepublished International patentapplication PCT/EP 97/03057 geogrids are disclosed where the transverseand the longitudinal straps are welded one on top of the other by meansof at least two welding zones per bonding point.

The invention will be further illustrated below with reference to anexample. It goes without saying that the scope of the invention is by nomeans restricted to said example.

EXAMPLE

The straps described below are welded one on top of the other with theaid of a solid state laser (OPC-A020-MMM-CS diode laser array) emittinglight at a wavelength of 820 nm. The optics in the welding set-up shapethe laser beam into a line 6 mm long. The distribution of intensity ishomogeneous over the length of the line. Over the width of the line thedistribution of intensity follows, approximately, a Lorentz distributionwith a full width at half maximum (FWAHM) of 0.3 mm. The total power ofthe laser light in the line is 15W. During welding the line is movedcrosswise at a velocity of 0.023 m/s across the plane to be welded. Thisresults in a continuous weld of 6 mm wide running the length of thescanning movement. If necessary, this process is repeated until thewhole contact area has been welded.

The scanning movement occurs parallel to the longitudinal strap.Consequently, when this strap is 12 mm wide, two scanning movementswhich do not overlap are needed.

Two types of transparent polyester transverse straps, “2cl” (averagecrack length: circa 80 mm; specific strength 636 MPa) and “5cl” (averagecrack length: about 30 mm; specific strength 631 MPa), having theproperties indicated in the Table, are each individually welded onto ablack polyester strap across the whole contact area and at an angle of90°, using the aforesaid laser. The black strap is composed of PET towhich carbon black has been added and has a specific strength of 631 MPaand a elastic modulus (longitudinally) of 13,8 GPa.

In a tensile tester 10 black straps with a “2cl” transverse strap weldedonto them and 9 black straps with a “5cl” transverse strap welded ontothem, respectively, are loaded to failure. The average decrease instrength of the black longitudinal straps provided with a “2cl” and a“5cl,” respectively, is listed in the Table below. ‘E_(tr)/E_(lg)’stands for the ratio of the crosswise elastic modulus of the transversestraps to the lengthwise elastic modulus of the longitudinal straps;“splitting” indicates whether prior to the failure of the blacklongitudinal strap there was any splitting in the relevant transversestrap.

TABLE Table 2cl (comparative) 5cl (invention) material drawn PET drawnPET thickness (in mm) 0.52 0.53 E_(tr) (GPa) 2.30 1.04 (E_(tr)/E_(lg)) ×100 (in %) 16.7 7.5 splitting yes No strength decrease (in %) 14.1 1.9

This shows that the decrease in strength and the attendant prematurefailure of longitudinal straps provided with “2cl” occur hardly if atall in the structure according to the invention (“5cl”).

We claim:
 1. A geogrid comprising drawn, polymeric longitudinal strapsthat run parallel or substantially parallel and polymeric transversestraps bonded to the longitudinal straps, characterized in that acrosswise elastic elastic modulus of the transverse straps is less than15% of a lengthwise elastic elastic modulus of the longitudinal straps.2. The geogrid of claim 1, wherein the crosswise elastic elastic modulusof the transverse straps is less than 8 % of the lengthwise elasticelastic modulus of the longitudinal straps.
 3. The geogrid of claim 1,wherein a crosswise elastic elastic modulus of the longitudinal strapsis less than 15% of a lengthwise elastic elastic modulus of thetransverse straps.
 4. The geogrid of claim 1, wherein the crosswiseelastic elastic modulus of the longitudinal straps is less than 8% ofthe lengthwise elastic elastic modulus of the transverse straps.
 5. Thegeogrid of claim 1, wherein when a strain is exerted on one or morelongitudinal straps of at least 90% of the specific lengthwise strengthof the longitudinal straps, the greater part of the transverse strapsbonded to said longitudinal straps will be deformed without cracking orsplitting.
 6. The geogrid of claim 1, wherein the longitudinal andtransverse straps are bonded together at an angle between 80° and 100°.7. The geogrid of claim 1, wherein the straps are bonded together bymelting a portion of the polymer of the longitudinal strap and thetransverse strap.
 8. The geogrid of claim 7, wherein the straps arebonded together by melting only 5 to 100 μm of the polymer of thestraps.
 9. The geogrid of claim 7, wherein the straps are bondedtogether by melting only 5 to 30 μm of the polymer of the straps. 10.The geogrid of claim 1, wherein at least the longitudinal straps aremade of a polyester.
 11. The geogrid of claim 10, wherein the polyesteris polyethylene terephthalate.
 12. The geogrid of claim 10, wherein thepolyester is a copolymer comprising ethylene terephthalic moieties. 13.The geogrid of claim 1, wherein one of the straps comprises aradiation-absorbent material that absorbs bond-forming radiation at apoint where bonding is to be effected between the straps.
 14. Thegeogrid of claim 13, wherein the radiation-absorbent material absorbsradiation having a wavelength of between 600 and 1600 nm.
 15. A civilengineering structure that has been reinforced with a geogrid accordingto claim 1.